The present invention pertains to turnbuckles and similar devices for tensioning stays, rigging, guides and the like. The present invention is particularly suited for use with sailing vessels.
Turnbuckles comprise the majority of devices presently used to control the tension to the stays used to position the masts of sailing vessels. The turnbuckle consists of a rigid member or housing with either screw threads at both ends or screw threads at one end and a swivel at the other end. The stay is attached to threaded bolts threadably engaged with the groove threads on the rigid member. A conventional mast for instance is held in position with four mutually orthogonal stays. Each stay is attached at one free end to the mast and at the opposite free end to the sailing vessel. A turnbuckle, generally with screw threads at both ends, is attached to each stay between the free ends. Thus, a total of four stays and four turnbuckles are used in most instances to position the mast. A standard turnbuckle has right handed threads at one end and left handed threads with the same pitch at the other end. The turnbuckle threads therefore may be either coarse or fine but not both. Therefore, rotation of the rigid member causes the threaded bolts to move in or out depending upon the direction of rotation. One complete rotation of the rigid member equals a shortening or lengthening of the turnbuckle equal to the movement caused by two complete rotations of either threaded bolt. If either a turnbuckle or a stay fails the loss of tension allows the mast to fall. When an overstressed turnbuckle fails, the threads are stripped or the housing breaks or possibly both. But in any event, a failed turnbuckle is of no further use and must be replaced entirely.
The weakest element of the turnbuckle is the threaded surface of the threaded bolts at either end. The weak spot of the turnbuckle is where the engaged threads of the bolts and the rigid member meet. Any excessive strain or stress is applied to the contact surface with the area of minimum diameter. Over tensioning stretches the metal which will eventually fail. However, before turnbuckle failure, metal stretch results in loosening of the stays and sagging or leaning of the mast since the separate turnbuckles do not deform uniformly. Subsequent readjustment of the turnbuckle may conceal the damaged portion within the threaded end of the rigid member. However, the damaged threads are still under constant tension even though concealed and thus are subject to further damage.
The threads of each turnbuckle assembly, the rigid member and threaded bolts, are also subjected to constant vibration transmitted through the stays. Occasionally harmonic vibrations are established which may be quite damaging to the threads. The damage to the threads due to vibration is usually concealed within the threaded portion of the rigid member since it is the threads in contact with one another that will be damaged by the vibration.
It is common practice to overcome the faults of the standard turnbuckle by over engineering the turnbuckle. This overengineering acts to transfer the location of failure from the turnbuckle to the stay or its associated hardware resulting in a more serious or dangerous uncontrolled failure.
What is needed is a device that provides the function of a turnbuckle without the separation and accompanying losses upon its failure. In other words a device that contains any failure that may occur.